When the display is flashing are you able to set the timer on there?
Ok that's fine. What its going to be is the timer itself. Theres no battery back up as the timer is powered by the mains power going in to it.
Now its a very common problem on these Bekos. Ive replaced loads of them for this fault with it alarming, Flashing all the time and not able to reset it or going dim.
To fix this you will need to replace the timer clock and that will sort it out. Its not that bad of a job if you are handy at a bot if diy.
Just pull oven out and make sure power is off.
Remove the top and you will see the clock there.
Remove the 2 screws holding it on and disconnect the wiring block as this is just a block with about 4 wires on there and you can swap it over from there.
Click here for the part
If you need anymore info just get back to me.